The use of Link Capacity Adjustment Scheme (LCAS) for virtual concatenated signals is described in the ITU-T Standard Recommendation G.7042. The Recommendation specifies a link capacity adjustment scheme that should be used to increase or decrease the capacity of a container that is transported in an SDH/OTN network using Virtual Concatenation. The scheme is applicable to every member of the Virtual Concatenation group.
This Recommendation defines the required states at the source and at the sink side of the link as well as the control information exchanged between both the source and the sink side of the link to enable the flexible resizing of this Virtual Concatenated signal. The actual information fields used to convey the control information through the transport network are defined in their respective Recommendations, which are ITU-T Recs G.707/Y.1322 and G.783 for SDH and ITU-T Recs G.709/Y.1331 and G.798 for OTN which are also called OTH—Optical Transport Hierarchy.
For example, in SDH/SONET frames, control information concerning LCAS protocol is transmitted by means of POH (Path Overhead) bytes of a standard SONET/SDH transport module. In OTH, such LCAS control information is normally transmitted in OPU-k virtual concatenation overhead (VCOH 1/2/3).
LCAS, in the virtual concatenation source and sink adaptation functions, provides a control mechanism to smoothly increase or decrease the capacity of a link to meet the bandwidth needs of the application. It also provides the capability of temporarily removing member links that have experienced a failure. The LCAS assumes that in cases of capacity initiation, increase or decrease, the construction or destruction of the end-to-end path of each individual member is the responsibility of the Network and Element Management Systems.
Synchronization of changes in the capacity of the transmitter (So) and the receiver (Sk) shall be achieved by a control packet. Each control packet describes the state of the link during the next control packet. Changes are sent in advance so that the receiver can switch to the new configuration as soon as it arrives.
It should be noted that LCAS protocol is transmitted between a source and a destination via the transport network as control information. Since transmission via the communication link is usually bi-directional, a so-called hand-shaking procedure is required to maintain the transmission. Therefore, the control information concerning LCAS protocol is transmitted by means of overhead bytes (such as POH bytes of SDH data frame), and in both directions of any bi-directional link.
In cases of substantially unidirectional data traffic, data flow in the opposite direction of a bi-directional link is almost negligible. Such situations are typical for video on demand applications, when, according to a customer's order, massive data flows carry the ordered video information in one transmission direction via the communication link. Another example of a unidirectional data traffic is any kind of e-learning process when heavy data traffic is transferred to a client (in one transmission direction) upon his/her single momentary request (in the opposite direction). In these cases, the need of transmitting LCAS in the mentioned opposite direction results in forwarding almost or completely empty SDH/SONET or OTN transport modules just for the purpose of transporting some informative overhead bytes thereof. Consequently, bandwidth capacity of the communication link is utilized non-effectively in such cases.